Combine harvesters thresh grain by passing crop material between a rotating rotor or cylinder and a stationary concave of a threshing assembly. The operator of a combine harvester can control the efficiency of the crop threshing assembly by changing the gap between the rotating rotor and the concave, changing the rotational speed of the rotor and by changing the quantity of crop material passing through the rotor and concave.
The operator controls the feed rate of crop material passing through the rotor and concave by controlling the ground speed of the harvester. Normally the operator attempts to adjust the ground speed to hold the feed rate at a level which will maximize harvester capacity without overloading harvester components.
Rotor speed and the gap between the rotor and the concave are two adjustments that are available to the operator to ensure that all the grain is threshed from the heads, that the quantity of grain that is cracked or otherwise damaged is minimized and that the quantity of material other than grain to be separated by the sieve and chaffer is minimized. These adjustments can be made from the operator's cab on some combine harvesters.
Adjusting the gap between the rotor or cylinder and the concave and the rotor or cylinder speed is to some extent an art, especially in unusual and difficult threshing conditions. Adjusting the gap between the concave and the rotor is a mechanical adjustment and generally remains fixed once it is made.
Adjusting rotor speed is a more difficult matter in that there are a number of factors that can change rotor speed. The internal combustion engine, that drives the rotor, rotates at an operating speed controlled by a governor. Governors are generally mechanical devices that react to changes in engine speed. Well designed and manufactured governors allow some variation in engine speed. This variation in engine speed results in rotor speed variations.
Threshing cylinders and rotors have been driven by chain drives and belt drives. The output of chain drives can be changed by changing sprockets. Changing sprockets in the field is time consuming and is not something that is undertaken frequently. Belt drives have employed variable speed drive sheaves that are adjusted from the operator's station. Belt drives in high torque applications have a limited operating life and high maintenance costs. For that reason it is desirable to drive the rotors of combines with axial crop material flow with hydraulic pumps and motors. It would also be desirable to employ a hydraulic pump and motor to drive a cylinder with tangential flow in a high capacity harvester. Hydraulic pumps and motors can be designed to deliver high torques and their output speed is easily adjusted. Hydraulic pumps and motors have generally not been used to drive rotors due to their cost and because the output speed of a hydraulic motor varies substantially depending upon the output torque and the viscosity of the hydraulic oil. The viscosity of the hydraulic oil varies substantially from cold mornings to high mid-day temperatures. Crops are harvested in some areas where temperature variations of 50.degree. F. or more are common during a 24-hour period. The volumetric efficiency of a new hydraulic pump will vary from 99% at low load and low oil temperatures to 94% at moderate to high loads and high oil temperatures. The hydraulic motor has the same change in volumetric efficiency. The change in the speed of a threshing rotor in an axial flow combine, with a hydraulic pump and motor drive having the volumetric efficiencies set forth above, will approach 14% of the high speed. The change in rotor speed, due to variations in engine speeds, may increase or decrease a change in rotor speed due to changes in the volumetric efficiency of the hydraulic pump and motor that result from load changes and temperature changes. A change in rotor speed of more than about three percent from the ideal speed would normally be considered unacceptable.